In order for two nodes in a radio communication system to establish a connection with each other, they need to be synchronized (aligned) in time and frequency. Such synchronization is typically achieved by having one of the nodes, e.g. an access node (AN), transmit synchronization pilot symbols, or pilots, which comprise a set of predefined signals on predefined radio resources. For example, the pilots may be transmitted on predetermined frequencies at regular time intervals. The other node, which may, for example be a user equipment (UE) node, may detect and use these signals as a reference to which it can aligns its own frequency reference and timing (clock). A “radio resource” may include a physical or logical resource that can be used to transmit information. A “radio resource” can include, for example, a frequency band in a frequency division multiplexing communication system, a time slot in a time division multiplexing communication system, a subcarrier of a symbol in an orthogonal frequency division multiplexing (OFDM) communication system, a spreading code in a code division multiplexing communication system, an antenna beam, etc.
The 3GPP Long Term Evolution (LTE) specification defines a primary synchronization signal (PSS) that is transmitted in the center frequency portion of an OFDM symbol every 5 milliseconds. The PSS is mapped into the first 31 subcarriers on either side of the DC subcarrier. Therefore, the PSS uses six resource blocks with five reserved subcarriers on each side.
FIG. 1 is a schematic illustration of an OFDM radio frame showing the location of the PSS. In frequency division duplex (FDD) mode, the PSS is mapped to the last OFDM symbol in slots 0 and 10. In time division duplex (TDD) mode, the PSS is mapped to the third OFDM symbol in subframes 0 and 5, as shown in FIG. 1.
As shown in FIG. 1, a secondary synchronization signal (SSS) is also transmitted. The SSS is based on maximum length sequences (m-sequences), which are pseudorandom binary sequences. Three m-sequences, each of length 31, are used to generate the synchronization signals. The SSS is transmitted in the same subframe as the PSS but one OFDM symbol earlier. The SSS is mapped to the same subcarriers (middle 72 subcarriers) as the PSS. The PSS and SSS are defined in 3GPP TS 36.211. “Physical Channels and Modulation.” 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA).
The division of the synchronization signal into PSS and SSS in the 3GPP LTE specification signals is designed to reduce the complexity of the cell search process.
In case of multi-antenna systems, the synchronization signals may additionally serve the purpose of finding suitable transmit and receive beam directions for communication between the two nodes. Different beam directions are then typically used for transmission on different pilot resources, allowing the other node to identify the best beam to use for transmission of signals.